Cement compositions are widely used in dental and biomedical applications. Typical dental uses include restoration of teeth by filling cavities following destruction by decay, cementing crowns, inlays and orthodontic devices in place, providing a base and/or lining in a tooth cavity etc. A wide range of cement compositions have been developed and are commercially available. These fall into a number of different types.
Glass-ionomer cements are acid-base reaction cements that typically set by the interaction of an aqueous solution of a polymeric acid with an acid-degradable glass,eg as disclosed in GB 1316129. The principal setting reaction is the slow neutralisation of the acidic polymer solution to form a polysalt matrix. The acid is typically a polycarboxylic acid (often polyacrylic acid) and the glass is typically a fluoroaluminosilicate. The setting reaction begins as soon as the components are mixed, and the set material has residual glass particles embedded in interconnected polysalt and silica matrices.
The advantages of glass-ionomer cements include their adhesion to tooth tissue, thus allowing the use of conservative techniques, and sustained fluoride release, thus imparting to the tooth an increased resistance to acid demineralisation.
However the disadvantages associates with glass-ionomer cements are that the immature cement is sensitive to moisture contamination and hence requires protection to ensure optimum final mechanical properties are achieved. The nature of the setting reaction means that the strength of the glass-ionomer develops with time, consequently the immediate strength of the glass-ionomer is not as high as that of other materials. Finally the glass-ionomer is not as tough as some other dental cements.
Composite resin cements set by the free-radical polymerisation of a resin (monomer) component. The cements usually include a non-active filler (usually a glass and/or silica) which is not involved in the setting mechanism of the materials, although the filler is generally bound into the matrix via a difunctional silane coupling agent. The monomers are generally large molecule aromatic or urethane dimethacrylates, selected with the aim of minimising polymerisation shrinkage of the material.
However these monomers have quite high viscosities and consequently smaller dimethacrylate monomers are used as diluents to lower the viscosity and thus increase the capacity for filler incorporation.
Composite resins are supplied as either one or two paste systems, depending upon the method used for initiation of the polymerisation reaction. The reaction may be initiated by an external energy source (one paste) usually high intensity blue light (470nm) eg using an .alpha. diketone with an amine reducing agent as the initiator system. Alternatively the polymerisation may be initiated by mixing the components (two paste) with eg a peroxide and tertiary amine as the initiator system.
The advantages of the composite resin are good aesthetics, good mechanical strength nd good wear resistance. However because of the nature of their setting reaction they have the disadvantages of polymerisation shrinkage, polymerisation exotherm, water sorption and monomer leaching. Shrinkage during curing is a particular problem because it allows microleakage around a restoration which can cause further decay of the tooth. It also means that stresses can be set up in the filling or the tooth.
Resin-modified glass-ionomer cements (RMGICs) were introduced with the intention of overcoming the problems associated with the conventional glass-ionomer, eg uncontrolled chemical set and tendency towards brittle fracture, whilst still retaining its advantages, eg fluoride release and adhesion. To achieve this the technologies of the acid-base and resin cements were combined. See, eg, EP 0323120, U.S. Pat. No. 4872936 and U.S. Pat. No. 5154762. One attempt to achieve this advocated simply replacing some of the water in a conventional glass-ionomer cement with a hydrophilic monomer. Another approach also replaced some of the water in the formulation, but in addition modified the polymeric acid so that some of the acid groups were replaced with unsaturated species, so that the polymeric acid could also take part in the polymerisation reaction.
Resin-modified glass-ionomers have two setting reactions: the acid-base reaction of the glass-ionomer, and the polymerisation of the composite resin. The monomer systems used in resin-modified glass-ionomers are not generally the same as those in composite resins. This is because the monomer must be compatible with the aqueous acid-base reaction of the glass-monomer components.
Resin-modified glass-ionomers have the advantage of improved aesthetics compared with conventional glass-ionomers but they also have the potential for fluoride release and the adhesion of the conventional material although it should be noted that some materials are supplied with a bonding agent similar to that used with composite materials. The fracture toughness of the resin-modified material is higher than that of conventional glass-ionomers and in some cases the resin-modified material is higher than that of conventional glass-ionomers and in some cases the resin-containing materials have higher strengths. However, because of the polymerisation reaction involved, resin-modified glass-ionomers have the disadvantages of polymerisation shrinkage and exotherm, water sorption and loss of free monomer. These disadvantages are far more of a problem than they are for composite resins because of the small toxic monomers currently used in resin-modified glass-ionomers.
Acid-modified composite resins (compomers) set by photopolymerisation of their monomer system. However, the systems include monomers with acid character not found in conventional composite resins. The filler in these materials is typically made up, at least in part, of acid-degradable glass as used in glass-ionomers. Consequently, in the presence of water, the monomer should be capable of undergoing a glass-ionomer type reaction with the glass. Unlike conventional and resin-modified glass-ionomer cements, compomers are supplied as one paste systems. To achieve this the water, essential for the acid-base reaction, is excluded from the formulation. Once in situ the cement will take up water.
This water could then initiate the acid-base reaction potentially permitting a glass-ionomer cement style sustained fluoride release from the material. The aesthetics of the compomers are good but their fluoride release rate is lower than that of a glass-ionomer. The bonding system supplied for use with the material assumes that the material will behave like a composite.
The present inventors have carried out experiments to assess alternative monomer materials for use in polymerisable cement compositions, and have discovered that good results are obtained by use of a mixture of monomers including an amount of tetrahydrofurfuryl methacrylate (THFMA).
THFMA is known for use as a monomer material in polymers for a number of purposes, including a composite resin cements for use as provisional or temporary crown-and-bridge resin (WO81/02022 and U.S. Pat. No. 4264489), for use in the construction of dentures, dental bridges and crowns and as bone cements (GB 2107341), and for use in compositions for promoting tissue repair (WO93/09819).
U.S. Pat. No. 5154762 and AU 46717/89 both concern resin-modified glass-ionomer cements employing polymerisable unsaturated organic compounds, particularly various acrylates and methacrylates. These documents refer to a large number of possible polymerisable compounds, including THFMA, but do not include illustrative examples of use of this material and there is no evidence that THFMA has hitherto been used in resin-modified glass-ionomer cements.
In experiments with THFMA, the present inventors have been unable effectively to polymerise 100% THFMA (as broadly disclosed in U.S. Pat. No 5154762 and AU 46717/89) under clinically relevant conditions, but have found that on inclusion of at least 5% by weight of a suitable secondary monomer with the THFMA, polymerisation does occur, and that such monomer mixtures are useful in polymerisable cement compositions for dental and biomedical applications.